Filter-amplifier combination



Aug. 29, 1933- H. cHlRElx FILTER AMPLIFIER COMBINATION Filed March so, 1929 @NIV/L Y Patented Aug. 29, 1,933

UNITED STATES FILTER-AMPLIFIER COMBINATIONV Henri Chireix, Paris, France Application March 3o, 1929, ASerin No. 351,424, and in France VApril 4, 1928 13 claims.- (clins- 171i .The art of electric lters has been known for a number of years. It is known that an electric lter consists of an assembly `of cells or units eithersiinilar or dissimilar, closed in'both-ldirec- 1 5 tions upon what has come to be called the charactity varies within wideliinits,"say,l between zero and infinity, ror, between Zeropand finite value, then to fall back to zero; or, `else between innity and a finite value, then to rise again to infinity.

It is as a consequence quite diiiicult, if not iiripossible, to operate and'y use filters of the known kind under the required conditions. n p i l' The object of the present' invention is tofprol lVide an amplier'having the ultimate properties the band to be passed, 'and' rapidly growing at? i tenuation outside these limits) though, 'for"'rea` VsonsV of its very natura/different both' iti-theoretical and practical regard from ltersfproperly socalled. i S

The said result is attained by choosingfor the output circuit of an amplifier tube an impedance han-(her"` l .i network or c i einafter to be designated uacnlstm, Oractlon of .these tubeg. .Y Y

The fundamental principles underlying f my `inyentiori'and the manner-of applying the'samel 19.0

, as the filter group), onepart 'of which is associi ated with and acts uponthe i grid circuit of a sec-V ond tube either directly (if the separate source of i supply for the different stages are acceptableh or through the well-known means of a vleak Aresistance, whenever the `samesupply sources are V,to be employed.; AFundamentally speaking the number of stages may b e unlimited, though in, practice 'fr' it isl found that threeare ample,,especially when (and this is themost interesting case) the lter groupsconstituting the platecircuitof the Various tubes are proportioned ina different manner-i` in p the AVarious stages. characteristics which' distinguish the arrangement here disclosed fromv theknown type of filter areas follows:

(l) lThe lter group works on an open circuit, g in other words, furnishesino V.energythe leakage Finally, when calculating the `latter it is. found that, inside the ,ltering range, this quan.-`

or band.

of a band lter (regularit'y'iof amplication inside resistance or leak which mayY have a value of seueral megohm's has only the objectof` determining lthe potential'of the grid ofthe following tube); y

hence, no characteristic impedancev is herejin- Volved; and therelationship betweenthe input p69 i andoutput energies of the group isinnitely high.l (2) `The internal dynamic resistance of-theainf pliiier tubeand the resistance `feeding the ,highpotential to the plate' (Fig. 1-) exercise a dente action owing to their reduced Value (parallel `re-*.65 sistance of these two elements); vThe Value of this resistance isl decisivefor the quality of the filtering process (regularity of am'pliicjationfin-v Aside the. filtering zone) and the width of ythebanvdv i passed or transmitted. This'resistance,1 however, ,7D

cannot be compared with the characteristic impedance offknown lters for the followingV two reasons: v. f

(a) Its value should be constant 'for all'frequencies, Whereas the characteristic impedance of alter varies inside' wide limits within` the 1"* filtering band, and becomes'a purely imaginary quantityinthe regiorisiof attenuation.

(b) Variations,Av even when quite small` in f V`alue re'sult,A on-the contrary, in 'considerable variations'in the width ofthe band passedzand inthe attenuation curve for the attenuated zone l In YknownY filters, on the other hand, the width of the filtering zonedepends only upon ,impedances andithe characteristic impedance i8? playsno part at all in the attenuated Zone. (3) It' is possible to `unite without'any'inconvenience,'or Without adoption of `special pre- A'cautionary means, ltering'vgroups o'f" different Q 'n`.fi,'ture ,`such!` association being always by lmeans U of afrhultiplef,electrode` tubegno `reflection hav# ing 'to be Texpected because of the vunilateral char,-

so as to provide a-structure embodying the invenin which, 1

Fig. yli'showsfthe arrangement of a .filter amplii190 fier comprising S three `filtering groups according.

`to this-invention, i.

Fig.'2 shows the electric equivalent ofia simple jfilterggroup, type I, y

Fig. 4 shows v,the electricV equivalent of'y acorn-1 my .fposite filtergroiip of the cut-pff? frequencytype (type 1I) I Figs-3and illustrate the attenuation 'Referring 'to Fig-1 @hegpfiteriiaig me through resistances 6.

The resistances 6 are preferably rheostats, in-` deed, the value of theseresistances governs, as will be seen later, the 'width of theband and the regularity of the current in the filtering zone.

7, 8, V9, 10 constitute a filter group (I) of type I with no cut-off frequency. It Aconsists of a series resonant circuit (capacity 7, inductance 8) tuned to the geometric medium frequency of the filter, this circuit being'ccnnected in series with another circuit (inductance coil 9, capacity'flO) parallel or anti-resonant, and likewise tuned to the geometric mean frequency of the filter.

- The potential'set vup acrossjthe terminals cf this circuit is fed to triode l1 by the well-known assembly comprising coupling capacity 12 and A`leakv resistance 13.k The vlatter should have a rather large value (say, several hundred thousand ohms or several inegohms) so as notto constitute an appreciable shunt for the circuit- 9.-10. lElements 7', 8, 9,'10, 14, 15 connected inythe output circuit of triode 11 constitute a filter group II, of type 1I, with cut-off frequency.

Ilhis group also comprises a series resonant circuitv 7-8 one parallel ory anti-resonant circuit 910.,`and va second similar circuit 11i-15,

,f each of .these kthree circuits being tuned to the geometric mean frequency of the filter,Y The potential set Vup vatthe terminals; or" the complex parallel or anti-resonant circuit 9-14`10-15 is transferred to triode 16 whose plate circuit ,con-

sists likewise of a filter group III of type I with 'Thi's group'consists ofelements similarV to those of group II designated by the same reference letters. Finally, sincek the characteristic of these filtersis for open-circuit operation, a further triode 17permits obtaining'at the terminals of resistance 18 the'amplified and filtered potential.

rIt will be understood that v'resistance 18 isV not indispensable, if the useful impedance permits the fio'w' ofthe direct current of the plate. "It

will also be understood that, although the 'same reference numerals have been used for electrical elements having identical functions, their numerthe frequency intervalcomprlsed between the extreme points Wherethe value of the function ical values or sizes may differ, indeed, must be different as will hereinafter be seen.

For instance, filter group HI will'preferably have to be designed for cut-off frequenciesdiifering from those of` group 1I, and the values of .I y

A.is equal to unity. Furthermore, is thefrethe parameters of the capacities'and inductances '7, 8, 9, 10, 1li, 15 willbe different from the three stages. Itis also obvious that the orderof the stages as well as their number .maybe modified.

' For instance, the'first filter group could have been K of the frequency cut-olf type.

18 could, finally, be replaced by choke-coils, and

these could be regarded as. practically infinite impedancesinside the filtering'zone. ItV must nevertheless be noted that, if Vthe variable reodes 2, 11,'16 so'asto makethe plate resistance of the critical value adapted to obtainthe de` vsired filter effect. rIt is evident that four-electrode tubes could beemployed (that is, two-grid vtubes ori screenegrid tubes) Resistances 6 and 1,924,397 plified and filtered is fed tov the grid-filament The reactions and interactions, taking place in the structure, andr determining the values of the different parameters of the inductances and ca pacities, are so involved as to make it desirable to use mathematical' formulae to elucidate the laws governing'the phenomena taking place in the structure, Vand particularly to facilitatethe design .of the electric wave filtering groups of this invention whereby anyone, skilled in the art, may construct thesame.

For this purpose, the filter group in type I may be represented mathematically by Fig. 2. where L and Crdenote elements 9 and 10 of Fig. 1 Y(inductance and capacity), and

dltional'elements m C and represent elements 14 and 15, Fig. 1 (nt is-Yan.105 integer). This group results in an attenuation curve having a general shape as shown in Fig.

5. In Figs. 2 and4,finallyR denotes the reduced resistance due tothe parallel connection of .regulating resistance 6, and the internal` dynamic reno sistance of. the ampliertriode. The denotations used show thatthe circuits, 7, 8, 9, 1 0', 14, V15 .are all tuned to the same frequency which shall be vdesignated by fo. i

Referring to Fig.` 5 which shows the attenuation 115 curve for the most general case of the filter group type II,it will be notedthat starting from fo where the ratio is unity, this relation both for increasing as well as for decreasing frequencies, begins to grow,

then crosses unity again, drops to zero, and then grows again toa liminal value.

The'passing or transmitted band shall be called quency interval separating two' maximum values; 5F, the frequency' interval rbetween two cut-off frequencies, v while f shall denote, generally lspeaking the frequency interval between two symmetricpoints about fo having the same value of V. I v l Fig. 5 shows three values of f found under *differentk conditions v (between,v the two maxima,

between the twocut-off frequencies, andbeyond the latter) In the Acase of Fig.l v3, Fis infinitely high so 'that the graph tends asymptotically towards Zero, for zerofand. infinite frequency.k It is finally easy tojdemonstrate mathematically that all of theabove frequencyintervals admit Vas a geometric medium frequency fo, inother words, all of theseintervals are delimited by two frequencies whose average' isfo. f

vio

"a formula which'contains in' the Vonly A and the Variablequantty between `V0.8 fand 2.v

mme? A mathematical study of the `filter group-type' I then leads to the following results: Denotingby the quantity in other words, the relationship between-resistance R and reactance Lm` of theA inductance L at 'pulsation m corresponding toirequency. fn, the

attenuation curve of Fig. 3 depends only upon quantity pia which shall be` designated' by defining quite correctly and definitelythe ltering curve; it is moreover'a simple number without dimensions. v ,i

If A is higher than, or equaltofZ, nopass band is present,

(DIQ

decreasing as lo' is departed from. Onthecon- .trary, if starting from A=2, the value of A falls off, an increasinglywider Yband isobtained, the two humps ofthe graph become farther removed and rise at the Sametime; inother wordsfth'e filtering range becomes wider and more efllcient.

Y* The maximum of alsA is givenA by the expression:

(9m-JMPA) .s K. and the value of the corresponding band isgiven by the expression:

f T=yl(2A)a` Where a and fo occur. Still the general expression which connects f Vand Y depends only upon 'A and has this form: v

second `member 9i.. fl, l By way of example, Formula; 1, 2 and3 show that Afor A=14 the practical mean value J is equal to 1.045 (Equation `(1) theampiitude is reduced to 0.2 for a band f=2.946f1.(Equation 4(3) II, moreover, LL-0.1 thebandpassedwillibe `laf y V` p s Taj-v0.25. y. 'The practical `values toybe admited for Afrange Indeed,f.1or A=0.8,-Formula (l) results in I ,e

" f Zarza" .-5 e

A thus turns out ot be a. parameter .in vvhich, apartmfromfthe variable af, there 'occur lonly thefknown frequency intervals vfi and 6F .{g'iven bythe problem)` and thefquantity A. e,

group I, that in alter amplifier designed in. ac-

yisf a. constant for. anishedlter amplifier, if R 1.,

and mpegs .that "there otherCwords, thehumps reach's high a'value as-25%. In the presence i''asingle stage `this value would still be acceptable, but it becomes prohibitiverapidly in the presence of a plurality lThemathematical-study of the lter group type II with #cut-off frequency `evidentlyisfar more complicated.:l Also, in this instance, it is possible to dene.;a.number A, though the-.latter yis no longer associated `inthe sameway with the .physif cal constants, but as vbefore itdefinesl the lter action and khas a range for all practical purposes;

thengiveni strictly byr ,this equation:

`between 0.8 and 2...; The graph shown in Figp'is .f I designates theY upper cut-off frequency` s corresponding to the limit of band 6F, and if '115 has thesame-denotation `and signicance asfbe-` lore,I determination 'of the electrical values tobe .chosen for these elementsfis possiblegassoon as A nhasbeen chosen on the basis lof the following re- Yi i fvg-(gy 1 It is eas'y to understandl that Formul, '7, bel ,come identical to` Formulf, 3, 2, and 24 `Whenever .aF rimasto' fanmsmmy high .valuein `'other words, when changing from the filter group typeIItotypeL 5 It can, also,be easilyseemrrespecting the" filter .1 ,y

cordance with-i thisdisclosureQthe valueofresistance R playssa ,very critical part.` Since is deflnedby the relation of capacities '7 and v10 and variesgi@variesandfas a consequence also A, since A -s'alyWhen A fvarie's, also `the Widthof the pass or` transmissible channelfisialtered (Equal "tient-2501. Usmgaginthefexamplejbeforecited.

fsf ,1s-mined 150 for R=5000 ohms; it ispossible tov draw up the `following tabulation:

fo A e ma-,X

5. 980 2. 0. 000 1. f y 5. 660 1. 8 0. 140 1. 005 5. 340 l. 6 0. 200 l. 02 5. O00 1. 4 0.245 1. O45 4. 630 l. 2 0. 273 l. 09 4. 230. l 1. 0.315 l. 16 3. 800 0. 8 0. 347 l. 25

This Vtrable'fshows .the 'inter-dependence of resistance R and the filtering'characteristics.V

Summarizingmy invention, it willthus be seen thatl haveV constructed and arranged a filter amplifier system having the characteristics of a band lter, and including tubes having three or more electrodes, the connection between the tubes Abeing established by means of filtering groups .comprising impedance net works or :chains con- Hnected in series with the. internalresistance of each tube. ZOptionally a regulatingA resistance is .connected from the Voutput circuitfof each tube between successive tubes as shown in the drawing. Two types of i'lltersare provided, and the internal resistances of the tubes and the regulating resistancesplay in these vtypes of filters a fundamental part. Another essential feature of this .type of filter system resides in the fact vthat the successive filter groups are closed upon a very -highi impedance, or even an-innitely high impedance, iny other words, that they furnish no energy. That is to say, when it is stated that a filter group does notdeliver any energy.. this fact is approxima'telyv true-since the. output' of the group is connectedbetween the gridand filament 0f a tube, it being noted that'the latterv'furnishs 'the energy through theanode battery'of the amplifier. f f

Although in' what precedes, the assumption has been made that all of the circuits were Atuned to fthe same frequency, it'may be of Vinterest or desirable in certain instances, to tune them or make lthem responsive to slightly different wavei-lengths so as to` insure certain yadditional effects, for instance, a slight change inthe position of a cutoff frequency.

While I have indicated and described only one system for .carrying my invention linto effect, it

will be apparent to one skilled in the art that my invention is` by no .means limited to the particular organization shown and described, but that many modifications in the circuit arrangement used, as Well as in the apparatus. employed,

may be made without departing from the scope of my invention as set forth in theappended claims.

WhatjI claimis: *l Y 1. In 'a thermionic'tube jampiiiier, at-least one Y filtering group comprisingiatubaa resonant circuit; composed of'a capacity andan inductance in ,series and connected between the plate of said vtube andthe grid of a second tube, rand acomplex 'anti-'resonant circuit composed "o f `two resonant branches 'in parallel, each'branchcomprising an inductance and a capacityininverse order, the

. inductance of each. branch -being .in resonance with the capacity-of the other branch, to the nat- :ural period of'the'anti-resonantcircuit,,this circuit being connected across the grid-filament space of thesaid vsecond tube. i v .i v

v 2, Ina thermionic,tube'hampliiier, atleast one A.filtering group comprising a tube,a resonant ,cir-Y cuit connected in series with athe internal resist- `ance of thertubeand a complex anti-resonant circuit composed, of two resonant branches lin parallel, each branchcomprising an inductance and a capacity in inverse order, the inductance of each branch being.r in resonance with the capacity of the other branch to the natural period of the anti-resonant circuit, this circuitV Vbeing .connected vacross a Very `high,imped ancethe complex. circuit `being connected with said vresoing group being tuned at Ythe same frequency equal to the geometric mean of frequencies of the filtering group. f

'3. In athermionic tube amplifier, at least one filtering group comprising a tube, a resonantcircuit in4 series with the internal resistance vof the tube, a resistance connected in parallel ,toA said internal resistance, and .a complex anti-resonant circuit composed of two resonant branches in,V

parallel, each branch comprising an inductance 4and a capacity in inverseorder, the 4inductance of .each branch beingV in resonancewith the yca pacity of the otherv branchft'o thejnaturalperiod of ythe antii-resonant circuit, this circuit'being .connected acrossfa very lhigh impedance ccn-` nected with said'resonantcircuit.

4Q In a thermionic tube amplifier, rat least one filtering group comprising a tube, a resonant circuit in series with the internal resistance of the tube, a variable resistance connected in parallel to said internal resistance, and a complex antiresonant circuit composed fof :two resonant `branches in parallel, each branch comprising an .inductance and a lcapacity in inverse order, the

inductance of each branch being `in resonance with the .capacityzof the otherbranch to the natural period of the anti-resonant circuit, thisV circuit being connected across a very high irnpedance connected with said resonant circuit.

5. In a thermionic tube amplifier, at leastV one filteringgroup` comprising a tube, al resonant cir- ;nant circuit,.,all circuits constituting fthe filtercuit in series with theinternal resistance. of the v 6. Inv a `thermionic,tubeamplifier, at least oneV filtering'group comprisingv a tube, .a resonant circuitvin series with the internal resistance of the tube, a complex. anti-resonant circuitconiposed of two resonant branchesvin parallel, each branch comprising an inductance and a capacity in infverse order, the inductance of each branch being.. i y

in resonance with the capacity of Vthe other nant circuit, thisY circuit, being connected across the grid-iiiament space of a second tube, each branch being tuned at filtering group. s f

. '7.A In a. thermionic vtubeemplifier, at. least -one lfiltering group comprising a tube, aresonantcircuitinseries'with the internal resistance of Vthe branchJr to the natural, period .ofthe anti-reso-V n I a cut-off frequency of the` tube, a complex anti-resonant circuit composed of two resonant branches 4in parallel, each branch comprising an inductance and a capacity in in- :'verse order, the inductance Yof each branch being inA resonance with the capacity ofgthe other branch to the natural period of the anti-resonant rcircuit, this'circuit being` connected across the gridfilament space. of a'. second tube, each-branch. ..I

being tuned ata cut-off frequency of the filtering Cil group, and the middle of the two cut-oi frequencies being equal to the geometric mean of frequencies of the ltering group.

8. A lter group comprising a space discharge tube, an ohmic resistance in the tube output, a tuned circuit consisting of an inductance and a capacity in series, and connected in series with the said ohmic resistance and a complex antiresonant circuit composed of two resonant branches in parallel, each branch comprising an inductance and a capacity in inverseorder, the inductance of each branch being in resonance with the capacity of the other branch to the natural period of the anti-resonant circuit, this circuit being connected in shunt relation to the terminals of said assembly.

9. In a filter, at least one lter group comprising an ohmic resistance, a resonant circuit constituted by an inductance and a capacity in series, and connected in series with the said ohmic resistance, and a complex anti-resonant circuit composed of two resonant branches in parallel, each branch comprising an inductance and. a capacity in inverse order, the inductance of each branch being in resonance with the capacity of the other branch to the natural period of the anti- Vresonant circuit, this circuit being connected in shunt relation with the terminals of the assembly, an impedance appreciably larger than the surge impedance of the lter being connected between the output terminals of the said group.

10. A iilter group comprising a thermionic tube, a highimpedance across the tube output, a resonant circuit constituted Yby an inductance and a capacity in series and inserted in the anode circuit of the said tube, and a complex antiresonant circuit composed of two resonant branches in parallel, veach branch comprising an inductance and a capacity in inverse order, the inductance of each branch being in resonance with the capacity of the other branch to the natural period of the anti-resonant circuit, this circuit being connected in shunt relation across the terminals of the group.

11. A lter group comprising a thermionic tube,

an ohmic resistance shunting the anode lament vspace of the said tube, a resonant circuit consisting of an inductance and-a capacityy in series and included in the anode circuit of the tube,r and a complex anti-resonant circuit composed of two resonant branches in parallel, each branch comprising an inductance and a capacity in inverse order, the inductanceof each branch being in resonance with the capacity of the other branch to the natural period of the anti-resonant circuit, this circuit vbeing connected in shunt relation across the terminals of the group.

12. A `iilter group, comprising a thermionic tube, a variable ohmic resistance shunting the anode lament space of said tube, a resonant `circuit constituted by an inductance and a capacity in series, and included in the anode circuit of the said tube, and a complex anti-resonant circuit composed of two resonant branches in parallel, each branch comprising an inductance nected in shunt relation across the terminals of i the group.

13. In a thermionic amplier, at least one iilterA assembly, comprising a thermionic tube, an ohmic resistance shunting the anodeiilament space of said tube, a resonant circuit constituted by an inductance and a capacity in series and connected in the anode circuit of this tube`,vand a complex anti-resonant circuit composed of two resonant branches in parallel, eachv branch' comprising an inductance and a capacity in inverse order, thek circuit being connected in shunt relation across the terminals of the assembly, the grid Vfilament circuit of another thermionic tube being connected acrossthe output terminals of the said as' y sembly. i HENRI CHIREIX. 

